Transparent and electrically conductive single wall carbon nanotube films

ABSTRACT

An optically transparent and electrically conductive single walled carbon nanotube (SWNT) film comprises a plurality of interpenetrated single walled carbon nanotubes, wherein for a 100 nm film the film has sufficient interpenetration to provide a 25° C. sheet resistance of less than 200 ohm/sq. The film also provides at least 20% optical transmission throughout a wavelength range from 0.4 μm to 5 μm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/583,545, filed Oct. 19, 2006, which is a continuation of U.S. patentapplication Ser. No. 10/622,818, filed Jul. 18, 2003, now U.S. Pat. No.7,261,852, which claims the benefit of U.S. Provisional Application No.60/417,729, filed Oct. 10, 2002, and U.S. Provisional Application No.60/397,254, filed Jul. 19, 2002, the disclosures of which are herebyincorporated by reference in their entirety, including any figures,tables or drawings.

This invention was made with U.S. Government support under Grant No.DAAD19-00-1-002 awarded by DARPA. The U.S. Government has certain rightsin this invention.

FIELD OF THE INVENTION

This invention relates to the field of carbon nanotubes, and moreparticularly, to uniform films of single-walled carbon nanotubes (SWNTs)which are electrically conductive and optically transparent.

BACKGROUND

Carbon has four known general structures including diamond, graphite,fullerene and carbon nanotubes. Crystalline structure refers to thelattice arrangement of atoms. Carbon nanotubes refer to tubularstructures grown with a single wall or multi-wall, which can be thoughtof as a rolled up sheet formed of a plurality of hexagons, the sheetformed by combining each carbon atom thereof with three neighboringcarbon atoms. The carbon nanotubes have a diameter on the order of a fewangstroms to a few hundred nanometers. Carbon nanotubes can function aseither an electrical conductor, similar to a metal, or a semiconductor,according to the orientation of the hexagonal carbon atom latticerelative to the tube axis and the diameter of the tubes.

Originally, carbon nanotubes were produced by an arc discharge betweentwo graphite rods as reported in an article entitled “HelicalMicrotubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp.56-58) by Sumio lijima. This technique produced mostly multiwall carbonnanotubes. A method of producing mostly single wall carbon nanotubes wassubsequently discovered and reported D. S. Bethune and co-workers(Nature, Vol. 363, pp. 605 (1993)).

There exist numerous applications for optically transparent,electrically conducting films. Since, as produced, single wall carbonnanotubes are known to contain a substantial fraction of intrinsicallymetallic nanotubes (typically about ⅓), nanotube films could be usefulin such applications, provided the films were optically transparent,possessed uniform optical density across their area and possessed goodelectrical conductivity throughout the film. Optical transparencyrequires the films to be made sufficiently thin. Uniform optical densityacross an optical aperture requires the nanotubes to be homogeneouslydistributed throughout the film. Finally, good electrical conductivitythroughout the film requires sufficient nanotube-nanotube overlapthroughout the film.

The principal problem in producing nanotube films which meet theserequirements of thinness, homogeneity and good intertube contact is thelack of solubility of the nanotubes in any known evaporable solvent.Given such a solvent, the nanotubes could simply be dissolved in adilute concentration and then cast or sprayed in a thin uniform layer ona surface, leaving behind the desired transparent nanotube layer oncethe solvent evaporates. Because no such solvent for the nanotubes isknown, if a deposition is attempted with nanotubes dispersed (e.g. byultrasonication) in a solvent such as ethanol, inhomogeneous clumps ofnanotubes result over the area of the deposited region.

Nanotubes can be uniformly suspended in solutions with the aid ofstabilizing agents, such as surfactants and polymers, or by chemicalmodification of the nanotube sidewalls. However, stabilizing agentsinterfere with the required electrical continuity of the nanotube film.Stabilizing agents are generally electrical insulators. Once the solventis evaporated, both the nanotubes and the stabilizing agent remain, thestabilizing agents interfering with the intertube electrical contact. Inthe case of chemical modification of the nanotube sidewall, theelectrical conductivity of the nanotubes themselves is degraded.

As a result, while thin and reasonably transparent films of nanotubeshave been produced for certain scientific purposes, such as forrecording optical transmission spectra, these films have not providedsufficient electrical conductivity necessary for applications requiringfilms which provide both high electrical conductivity and opticaltransparency, such as for optically transparent electrodes.

SUMMARY OF THE INVENTION

An optically transparent and electrically conductive single walledcarbon nanotubes (SWNT) film comprises a plurality of interpenetratedsingle walled carbon nanotubes SWNTs, wherein for a 100 nm film the filmhas sufficient interpenetration to provide a 25° C. sheet resistance ofless than 200 ohm/sq. The film also provides at least 20% opticaltransmission throughout a wavelength range from 0.4 μm to 5 μm. In apreferred embodiment, a morphology of the film comprises stacked planes,the SWNTs having random orientation in the planes. The opticaltransmission can be at least 30% from 0.4 μm to 5 μm.

The SWNT film can include at least one dopant. In this embodiment, for a100 nm thick film the sheet resistance is generally <50 ohm/square. Thedopant can selected from the group consisting of halogens and graphiteintercalants, such as alkali metals. The film generally consistsessentially of (>99% per weight) SWNTs.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application in publication withthe color drawing(s) will be provided by the Office upon request andpayment of the necessary fee. A fuller understanding of the presentinvention and the features and benefits thereof will be accomplishedupon review of the following detailed description together with theaccompanying drawings, in which:

FIG. 1 illustrates a scanned image of a transparent 300 nm thick SWNTfilm stretched across a hole on an aluminum plate, the film formedaccording to a preferred method for forming films according to theinvention.

FIG. 2 illustrates the transparency and clarity of a thinner (about 90nm), but larger diameter film as compared to the film shown in FIG. 1mounted on a plastic sheet.

FIG. 3 illustrates the transmission spectrum for a 50 nm SWNT film on aquartz substrate and a 240 nm thick freestanding SWNT film formedaccording to an embodiment of the invention, for both doped and de-dopedfilms, showing high transmittance throughout the visible and the NIRrange. The freestanding, thicker, film allowed recording of thetransmittance over the broad spectral range without interferingabsorption from a supporting substrate.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides single wall carbon nanotube (SWNT) films whichsimultaneously exhibit high electrically conductivity, opticaltransparency and uniform optical density across their area, and methodsfor producing these films. Films according to the invention transmitlight in both the visible and infrared portion of the electromagneticspectrum.

Bulk electrical conductivity in nanotube aggregates requires that someappreciable fraction of the nanotubes be electrically conducting andthat this fraction have sufficient intimate electrical contact with eachother to transport charge throughout the bulk. Electrical conductivityof the nanotubes themselves can come from two sources. A first source isfrom the metallic nanotubes in the sample, which comprise about ⅓ of thenanotubes in SWNT material obtained commercially. A second source cancome from semiconducting nanotubes in the sample, provided thesemiconducting nanotubes are doped with a suitable charge transferspecies. For example, halogens such as bromine and iodine, or alkalimetal atoms, as well as certain other atoms or molecules, can be used ascharge transfer species. Bulk electrical conductivity of the film ismaximized by a high degree of nanotube contact, as well as the nanotubesurfaces being largely free of any residual stabilizing agent sincestabilizing agents tend to be electrically insulating materials.

Films according to the present invention are generally essentially purenanotube films, defined herein as films having at least 99% nanotubes byweight. However, the present invention includes nanotube composites,which can include a lower percentage of nanotubes, such as 80 or 90% byweight nanotubes, along with one or more generally electricallyconductive materials.

The film thickness can be tailored to range from a few tens ofnanometers to several micrometers. The films produced using theinvention have a substantially uniform nanotube density across theirarea which results in optical clarity. Optical transparency is enhancedfor thin films (e.g. nm) as compared to thicker films (e.g. 3 μm). Filmthicknesses in the upper range generally become opaque. Particularly fortransmission into the IR, optical transparency is believed to beenhanced by a low nanotube carrier density.

A preferred method for forming electrically conductive and opticaltransparent SWNT films which exhibit uniform optical density acrosstheir area includes the step of dispersing a low concentration of SWNTs,such as 0.005 mg/ml, in a solution, such as an aqueous solution,containing a sufficient concentration of stabilizing agent to suspendthe nanotubes. Commercially available single walled carbon nanotubes,such as from Carbon Nanotechnologies Incorporated, Houston, TX provideroughly ⅓ metallic nanotubes and ⅔ semiconducting nanotubes. Preferably,the nanotubes used are purified to remove the large catalyst particleswhich are utilized in their formation.

The stabilizing agent can comprise a variety of surfactants such assodium dodecyl sulfate (SDS) and TRITON X-100™, or surface stabilizingpolymers. TRITON X-100™ is manufactured by the Dow Chemical Corporation,MI (formerly the Union Carbide Corporation). TRITON X-100™ isoctylphenol ethylene oxide condensate and is also referred to asOCTOXYNOL-9™. This material has a molecular weight of 625 amu.

In this preferred method, the SWNT solution is then applied to a porousmaterial. The porous material preferably comprises a filter membranematerial, such as polycarbonate or mixed cellulose ester. The filtermembrane preferably provides the following features:

1) a diameter larger than the optical aperture desired,

2) a high volume of porosity with a plurality of sub-micron pores, and

3) a composition that permits removal of the membrane material withoutdisruption of the thin SWNT film, such as through dissolution of themembrane material in a solvent or digestion of the membrane material inan acid.

The solution is then removed leaving the nanotube film deposited on themembrane surface. The resulting nanotube film is generally quiteflexible. In one embodiment, the solution is vacuum filtered off, withthe SWNT film formed on the filter membrane surface. Any remainingsurface stabilizing agent (e.g. surfactant) can be subsequently washedaway and the film can then be allowed to dry.

Good dispersion of the SWNTs in the original solution combined withfiltration provides a high degree of interpenetration of the nanotubesin the resulting film. This results because the nanotubes tend to remainstraighter, having a longer persistence length in the solution when welldispersed.

The first bundles to land on the flat filtration membrane surface areforced to lie essentially parallel to the surface. Because the filmgenerally grows at a uniform rate (with nanotube bundles lying acrossthose deposited before them), subsequently deposited bundles take on thesame planar orientations. The result is a film morphology wherein thenanotubes have random in plane orientations, but lie in stacked planes,with two-dimensional anisotropy similar to a biaxial oriented polymerfilm.

As a result, in the resulting film the nanotubes preferentially tend tolie across one another when forced during the filtration process ontothe membrane surface. This leads to films having improved electricalcontinuity and better mechanical integrity over other available methodsfor generating thin nanotube films. Moreover, the filtration results infilms having a high degree of compositional, structural and thicknessuniformity, which translates to a high degree of optical uniformity andclarity. The optical uniformity and clarity requires that variation inthe film thickness averaged over regions that are half the wavelength ofthe visible radiation be small. Such variations are within 10% for thefilms according to the invention.

In most applications once the nanotube film is formed on the porousmaterial, such as a filtration membrane, the film must be removed fromthe typically opaque porous material using a suitable method. Forexample, one of the following exemplary methods may be used:

1) Membrane dissolution can be used. For example, for mixed celluloseester membranes (MCEM), the membrane can be soaked in acetone ormethanol, which dissolves the membrane leaving the nanotube filmfloating in the solvent. To ensure minimal cellulose ester residue onthe film surface, the film can be transferred to fresh solvent, beforebeing laid on a second layer for drying. The surface tension of thedrying solvent ensures intimate contact between the nanotube film andthe selected layer once they are dry.

2) The nanotube side of the membrane can be pressed against a selectedlayer. To aid in obtaining intimate contact between the selected layerand the nanotube film, a small quantity of a solution that does notdissolve the membrane, such as purified water, can be placed between theselected layer and the nanotube film using surface tension to bring thetwo respective surfaces into intimate contact. The assembly includingthe membrane, the nanotube film and the selected layer can then beallowed to dry. The membrane can then be dissolved in a solvent in whichit is soluble leaving the nanotube film disposed on the selected layer.

A separation step is not necessarily required. For example, if theporous material selected is optically transparent in the wavelengthrange of interest, a separation step will not generally be necessary.

A possible limitation of the above-described method is that the filmarea can only be as large as the vacuum filtration apparatus provides.This is generally not a major limitation since such an apparatus can bemade arbitrarily large, or alternatively, the films can be formed usinga continuous process as described below.

In an alternate embodiment, a continuous process is used. In onecontinuous process embodiment, the filtration membrane can roll off aspool on one side of a vacuum filtration fit and be wound up, with thenanotube film, on the other side of the fit. The filter frit can have arectangular shape, the width of the membrane in its longer direction,but narrow in the direction of travel of the membrane. The filterfunnel, with its lower opening matching the fit, containing the nanotubesolution can sit over the frit, with a magnetorheological fluid to makethe seal between the funnel and the membrane moving by underneath.Keeping the frit narrow can reduce the force due to suction on themembrane, allowing it to be more easily drawn through the device. TheSWNT film thickness can be controlled by the SWNT concentration insuspension and the rate of travel of the membrane.

Thus, the invention provides methods for forming electrically conductiveand optically clear SWNT films. There are other presently availabletransparent electrode film deposition techniques for making non-SWNTfilms which can cover large areas and are compatible with thin filmprocessing technologies for making displays, solar cells and similardevices. However, these transparent electrode film deposition methodsare technologically demanding as they require expensive high vacuumequipment. Accordingly, a significant advantage of the invention is thatoptically transparent and electrically conductive SWNT films can beformed without the need for expensive high vacuum equipment.

SWNT films formed using the invention exhibit high mechanical integrity,including a high degree of flexibility. One advantage of high mechanicalintegrity is that the SWNT films can be made freestanding, provided thatthe film has sufficient thickness. Above some thickness, which dependson the optical aperture desired, the films can be made freestanding.Freestanding films provide a clear aperture free of any supportingsubstrate.

For example, a 240 nm thick freestanding SWNT film has been demonstratedover a 1 cm² aperture. Such a film can be supported on a framecontaining a hole, which when coated by the transparent nanotube filmcomprises an optically clear aperture.

FIG. 1 shows a back lit SWNT film according to the invention stretchedacross a hole on an aluminum plate. The film was produced as describedabove by vacuum filtering an aqueous SWNT/1 wt % Triton-X-100™surfactant solution onto a porous mixed cellulose ester membrane. Oncethe surfactant was washed away using deionized water, the film wasdried. The membrane was then clipped to the aluminum plate with the SWNTfilm contacting the plate. The assembly was subsequently dipped into anacetone bath, dissolving the membrane and transferring the film to theAl plate. The hole in the aluminum plate had a 0.59 cm diameter. Thefilm can be seen to provide a high level of transparency and opticalclarity to visible light.

FIG. 2 is a scanned image demonstrating the transparency and clarity ofa larger diameter, but thinner (˜90 nm) SWNT film mounted on a plasticsheet, such as a biaxially-oriented polyethylene terephthalate (boPET)polyester film marketed as MYLAR®. The resistance of a similar filmmeasured on an electrically insulating support (MYLAR®) exhibited asheet resistance of about 35 ohms/square when acid doped and about 175ohms/square when de-doped. De-doped films generally provide greatertransmittance in the IR as compared to doped films. This is a very highelectrical conductivity given the optical transparency, particularlysince high levels of optical transmission were found to continue wellinto the IR portion of the electromagnetic spectrum.

FIG. 3 shows the transmission spectrum experimentally obtained for 50 nmand 240 nm thick SWNT films formed according to an embodiment of theinvention showing high transmittance in both the visible and the NIRrange. For the 50 nm thick film, the surface resistance when acid dopedis seen to be about 60 ohms/square and when de-doped by baking to 600°C. in inert gas (gray spectral curve) is about 300 Ohms/square.Significantly, it is noted that the transmittance spectrum for the doped50 nm thick film is seen to be 70% or greater over the visible spectrum(0.4 to 0.75 microns). The spectral differences shown between the bakedand unbaked film arise because nitric acid, which was used in thenanotube purification, charge transfer dopes the nanotubes, while thebake-out desorbs the dopant species, dedoping the nanotubes. Such dopingalso impacts the electrical conductivity of the films making theelectrical conductivity for the doped films more than three times theconductivity of the de-doped films at room temperature.

High transmittance into the IR is useful because the 3-5 micron range isgenerally free of atmospheric water absorbance and is commonly used foratmospheric transmissions. Although there are a number of opticallytransparent, electrically conducting oxides materials available whichare useful in the visible part of the spectrum, in the IR, the number ofavailable materials retaining good transparency and electricalconductivity diminishes dramatically. Most conducting materials becomeless transmissive above about 2 μm due to what is generally referred toas “free carrier absorption”.

The depth of transparency into the IR is believed to be primarilylimited by the free carrier absorption of the metallic nanotubes whichmake up about ⅓ of the film. Should pure or increasingly semiconductingSWNTs become available, the films can remain optically transparentfurther into the IR, such as to a wavelength of at least 40 μm ascompared to more metallic nanotubes. As noted above, semiconductingfilms can be made electrically conducting by charge transfer doping,such as bromine or iodine doping.

The absorbance on the short wavelength side of the peak labeled M1 inFIG. 3 is due to many combined interband transitions. A characteristicfeature of the nanotubes is a sharp Van Hove (VH) singularity structurein the electronic density of states. The absorbance feature labeled M1arises due to transitions from the highest occupied valence band VHsingularity to the lowest empty conduction band VH singularity for themetallic nanotubes in the sample. The absorbance features labeled S1 andS2 arise due to transitions from the highest valence VH singularity tothe lowest conduction VH singularity and the second highest to thesecond lowest valence-to-conduction band VH singularities for thesemiconducting nanotubes in the sample, respectively. The absorptionjust beginning at a wavelength of about 2.4 μm in the unbaked sample andat a wavelength of about 4 μm in the baked sample is believed to beascribed to free carriers.

The charge transfer doping by the acid is a hole dopant meaning thatelectrons are removed from the nanotubes and transferred to the dopantmolecules which function as electron acceptors. This depletion of thevalence band electrons from the VH singularities results in the smallerabsorbance feature seen at Si in the unbaked (doped) versus the baked(dedoped) spectra shown in FIG. 3. Another consequence is the enhancedfree carrier absorption in the IR for the doped relative to the undopedcase, arising because of hole carriers injected into the semiconductingnanotubes. This provides evidence that the absorbance seen forwavelengths above 4 microns in the baked (dedoped) sample is largely dueto the free carrier absorption in the metallic nanotubes alone. Thus, aSWNT film comprised of only semiconducting nanotubes would betransparent much further into the IR.

Without metallic nanotubes in the SWNT film a loss of electricalconductivity would result. However, as noted above, semiconductingnanotubes can be doped to become more electrically conductive. Thus, forsemiconducting SWNT films, electrical conductivity can be enhanced bycontrolled doping with some accompanying loss of the additional depth oftransparency into the IR. Such controlled doping could be effected byexposure of the nanotubes to other air stable hole dopants besidesnitric acid, such as vapors of bromine or iodine. Alternatively, if thefilm is protected from atmospheric water and oxygen, electron donordopants, such as the alkali metals could be used. Even with SWNT filmsformed from presently available nanotube sources which include about ⅓metallic nanotubes, doping provides some measure of control over thetransparency and electrical conductivity of the resulting films.

SWNT films produced using the invention can be used for a variety ofapplications. For example, SWNT films formed using the invention can beused for solar cells, video displays, solid state light sources,receivers, or applications requiring an electrically conductive layerwhich is also optically transparent.

The SWNT films formed using the invention provide at least twosignificant advantages over conventional optically transparent electrodematerials. First, the SWNT films provide good optical transmission inthe 0.4 to 5 μm spectral range as well as high electrical conductivity.Second, the films formed are compatible with many other materials, suchas upcoming polymer active layers in a wide variety of devices. Apossible additional advantage for some applications is that by obtainingpurified SWNTs or adding a purification step, SWNTs essentially free ofmetal catalyst(s) can be provided for filtration according to theinvention, and as a result, resulting films according to the inventioncan tolerate 450° C. in air or over 1000° C. in inert atmospheres.

Most available optically transparent electrode materials requiretemperatures greater than 200° C. to fabricate. Because most polymerscannot tolerate such temperatures, the transparent electrodes must beproduced separately, for example, on a separate substrate, followingwhich, the active polymer is applied. Using the invention, the nanotubefilms in contrast, can be disposed directly on these polymer layers.

There are also several applications where the SWNT films can providesignificant advantages. One example is in transparent spectrochemicalelectrodes, where the inertness of the nanotubes may provide addedadvantages.

Optical modulators may also be formed based on the thin SWNT filmsproduced using the invention. For example, the SWNT film can provide oneelectrode of a capacitor like device consisting of indium tin oxide(ITO) on glass covered with a thin aluminum oxide layer covered with thethin transparent SWNT film. By applying a voltage between the ITO andSWNT electrodes, the SWNT film charges slightly, thus changing itsoptical transmittance over a particular absorption band of the SWNTfilm.

The invention can also be used to form chemical sensors. For example,the optical properties of the SWNT films can change in the presence ofhalogens or alkali ions, or possibly other species. It may be possibleto distinguish the presence of particular species from others throughidentification of particular resulting optical properties of the SWNTfilm in the presence of particular species. For example, by monitoringtransmission levels through a SWNT film formed using the invention, thepresence of certain chemicals can be detected. One advantage of theelectrical conductivity of the films in such applications is that bydriving sufficient current through them they can be self heated,desorbing the chemical species after it has been detected. Suchsensitivity recovery is enhanced by making the film freestanding overthe optical aperture allowing efficient self heating at lower current.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description as well as the examples which follow are intendedto illustrate and not limit the scope of the invention. Other aspects,advantages and modifications within the scope of the invention will beapparent to those skilled in the art to which the invention pertains.

1. An optically transparent and electrically conductive single walled carbon nanotube (SWNT) film, comprising a plurality of single walled carbon nanotubes (SWNTs), wherein the film has uniform nanotube density across said film's area with the nanotube's surfaces being free of residual stabilizing agents allowing intimate electrical contact between the nanotubes throughout said film, and the film is less than 1,000 nm thick, and wherein at a thickness of 100 nm the film has sufficient interpenetration to provide a 25° C. sheet resistance of less than 200 ohm/sq and at least 20% optical transmission throughout a wavelength range from 0.4 μm to 40 μm.
 2. The SWNT film of claim 1, wherein the SWNTs about ⅓ or less metallic SWNTs.
 3. The SWNT film of claim 1, wherein the SWNTs include at least one dopant, wherein for the 100 nm thick film the sheet resistance is <50 ohm/square.
 4. The SWNT film of claim 3, wherein the dopant is selected from the group consisting of nitric acid, alkali metals, halogens and graphite intercalants.
 5. An optically transparent electrode comprising the SWNT film of claim 1, wherein the electrode resides in a device.
 6. A chemical sensor comprising the SWNT film of claim 1, wherein the SWNT film detects the presence of a specific chemical by change in an optical property of the SWNT film upon absorption of the chemical.
 7. The chemical sensor of claim 6, wherein the sensor is reversible by desorbing the chemical from the SWNT film by driving a sufficient current through the SWNT film to heat the SWNT film.
 8. The chemical sensor of claim 6, wherein the SWNT film is free-standing in an optical aperture.
 9. An optically transparent and electrically conductive single walled carbon nanotube (SWNT) film, comprising a plurality of single walled carbon nanotubes (SWNTs), wherein the film has uniform nanotube density across the film's area with the nanotube's surfaces being free of residual stabilizing agents allowing intimate electrical contact between the nanotubes throughout the film and the film is less than 1,000 nm thick, wherein at a thickness of 100 nm the film has at least 20% optical transmission throughout a wavelength range from 0.4 μm to 200 μm, and wherein the SWNTs are undoped and about ⅓ or less metallic SWNTs.
 10. The electrode of claim 5, wherein the electrode is a spectrochemical electrode.
 11. The electrode of claim 5, wherein the device is a capacitor.
 12. The electrode of claim 11, wherein the SWNT film changes its optical transmittance over a particular absorption band of the SWNT film upon application of a voltage across the capacitor.
 13. The electrode of claim 11, wherein the capacitor further comprises a second electrode consisting of indium tin oxide (ITO) on glass with a dielectric layer between the electrodes.
 14. The electrode of claim 11, wherein the dielectric layer is an aluminum oxide layer.
 15. The electrode of claim 5, wherein the device is a solar cell, video display, solid state light source or receiver.
 16. The electrode of claim 5, wherein the device further comprises a material that limits the temperature to 200° C. during use and/or fabrication of the device.
 17. The electrode of claim 16, wherein the material is a polymer.
 18. The electrode of claim 17, wherein the polymer is polyethylene terephthalate. 